There are times when it is desirable to extend the duration of a laser pulse, with the extended pulse having a substantially lower peak power than the original pulse but with substantially the same average power.
One example of such need is the project in which the present invention was made. In particular, this project seeks to improve the ability of ground-based telescopes to observe galaxies and other celestial objects with much greater clarity and with a high resolution that can only be equalled by telescopes in satellites orbiting the earth above the atmosphere.
Approximately 60 miles above the Earth, there is a sodium layer formed by the remnants of tiny meteorites that continually burn out high in the Earth's upper atmosphere. A laser beam, turned to the sodium D.sub.2 line at 589 nm and pointed up at a desired point in the layer, will create an artificial "star" at that altitude by pumping the sodium atoms struck by the beam to a higher energy level, with light being emitted as the atoms return to their prior energy state.
Telescopes have been developed with 32-inch mirrors made up of many tiny flexible segments, each controlled by computer so that they can be instantly adjusted to remove distortions caused by atmospheric turbulence. Focused on the laser-lit guide star, 60 miles up, the segments of the telescope mirror will shape themselves to view the guide star clearly through the intervening atmosphere. Then, when the guide star vanishes as the laser beam is turned off, the finely tuned telescope will view the celestial objects with the same compensation for atmospheric disturbances.
Dye lasers, tunable to the sodium D.sub.2 line, and developed originally for research on anti-satellite weapons, have an average beam power of several kilowatts, sufficient to create an artificial guide star in the sodium layer. However, such lasers generate very short pulses, in the order of 60 nanoseconds, and with very high peak power, in the order of 10.sup.10 watts, and will essentially burn a hole through the sodium layer rather than causing the desired stimulated emission from the layer.
In order to make these lasers usable for creation of an artificial guide star it was necessary to find a way to reduce the peak power of the laser pulses without substantially reducing the average power in the laser beam.